The Commuting sub-forum has some subscribers who don't always read this forum.
I think I have some idea of where you're coming from. I'm finishing my Master's and getting married in March and April respectively, and at this point, I don't know what happens after that... but I've still worked out the coming season's training plan.
As far as specifics of your bike commute - I agree with the others. Wait until you're stationed. There are a few reasons. You don't know how far from base you will be, you don't know what kind of route you'll commute on, and it's a pain in the butt to travel with a bike. I guess you get some awesome deal on shipping, but even so - probably not worth it with your first station coming up so soon.
For myself, I've commuted on a singlespeed and currently commute on a multispeed road bike. Singlespeeds are nice because they're cheaper to build up and maintain, but I live in a hilly city now and prefer to have gears. I've also commuted on a mountain bike - that was when I was in college and had access to trails right around where I was commuting. So your existing mountain bike, or an old beater you have around, is another option. Hybrids suck. If you're already an athletic rider, you'll just be fighting it trying to get it set up for you. So either a proper road bike, geared or not, or a proper mountain bike, geared or not.
As far as the bag - I prefer messenger bags on road bikes and backpacks on mountain bikes. I find that backpacks always move to the side on a road bike, and I also ride in a lower position; backpacks end up too high on my back and put extra weight over my hands. Lame. I adjust my messenger bag to sit lower, nearer to my hips. But on mountain bikes, I sit a little more upright and feel more of the weight hanging from my shoulders, so it balances better with a backpack, and if I'm also mountain biking, I want something more secure. While a proper messenger bag won't fall in front of me - that's the purpose of the stabilizer strap - when I have them sitting lower, they can still end up someplace weird if I'm moving around enough.